Television image projection system



March 17, 1942. VON ARDENNE 2,276,750

TELEVISION IMAGE PROJECTION SYSTEM Filed March 18, 1940 SYNCHRONOUS MOTOR 40 3 TOALTERNATING CURRE/VTSUPPLY INVENTOR MANF'RED vo/v ARDEN/Vi ATTORNEY Patented Mar. 17, 1942 TELEVISION IMAGE PROJECTION SYSTEM Manfred WI! Ardenne, Berlin-Lichterfelde,

Germany 1 Application March 18, 1940, Serial No. 324,540 In November 30, 1938 scum (or. 178-75) This invention relates to television image projection systems wherein optical means are provided for projecting a television image upon a viewing screen. Such television projection tubes are shown and described in my applications Serial #292,01'7 filed August 26, 1939; Serial #306,;- 610 filed November 29, 1939; Serial #806,611 flied November 29, 1939; and Serial #324,539 filed is advisable to arrange the same in a plane in the path of the projected light beam in which the cross section of the beam is especially small;

March 18, 1940. In the tubes shown in these applications, in contrast to the hitherto customary television projections, the image is sustained uniformly as a whole on the projection screen. The disappearance or absenceof each individual line of the entire image occurs during a brief time period only, as compared. with the customary duration of the individual lines, which, for instance, maybe only 5 to of the duration of the frame image. Despite the briefness of this period of darkness in the devices disclosed in .my applications, a flickering is noticeable especially where the field or frame frequency is low (for instance 25 and less) This condition'persists even though the period of darkness does not occur simultaneously over the entire image but exists inthe form of a moving dark band over the image field in accordance with the formation of the image in a line by line manner. 'The elimination of this disturbance is of particular importance because with such elimination the operation can be car-.- ried out with the smallest number of images or frames per second which is possible to reproduce the movement so" that an especially narrow frequency band of the transmission channel will be suflicient. Furthermore, lower carrier frequencies may be used.

It is proposed to eliminate the disturbance due to flickering by introducing an intermittent projection similar to that in ordinary moving picture projectors. In accordance with experience gained in selecting the rotational speed of the shutter in movie projectors the interruption frequency of the projected light beam should be at then by a corresponding proper choice of the size of the sectors of the rotating. shutter relatively :brief periods of darkness may be attained. De-

spite the fact that the darkening of the image caused by the extinction period differs in character from the darkening of the image due to the rotating shutter or similar means, which cause the darkening of the entire image, the human .eye translates the effect into an elimination of the disturbing flickering.

It is possible in principle to maintain the shutter interruptions of the light in synchronism with the frame or image-frequency. However, a more favorable physiological impression appears if the periodicity of the shutter interruptions of the light beam represent a whole number proportion with respect to the number of images per second.

A great advantage of modern electronic television receivers and also of the storage projection receivers resides in the fact that in these receivers moving mechanical masses are entirely eliminated and consequently these receivers operate without any noise. Hence, the aim will be to substitute for the rotating shutter serving for interrupting the light beam, a means which operates in apurely electrical fashion.

Principally all sorts of light relays capable of controlling large quantities of light can be considered for this purpose as well as all controllable sources of light having a high total intensity.

passes through zero. In addition there exists the advantage of a particularly high light intensity and the further advantage that the periods of darkness represent a whole number proportion in respect to the image alternation cycle or frame frequency.

The invention may best and most completely be understood by referring to the drawing, wherein like reference characters represent like parts, and wherein:

Figure 1 represents one form of the invention in which a rotating shutter is used;

Figure la shows the detailed construction of the shutter Figure 2 shows another form of the invention wherein a gas discharge tube is used, and

Figure 2a shows the detailed construction of the gas discharge light source.

Figures 1 and 2 of this invention are more or less identical with the showing of Figures 3 and 4 respectively, in application Serial #324,539, re-' ferred to above. The Figures 1 and 2 of this invention include, however, means for interrupting the light beam at a frequency in excess of the television frame frequency.

Referring now to Figure 1, a television receiving tube 22 is shown whichincludes a crystal or relay electrode 32, one side of which is adapted to be scanned by a beam of electrons 28. -A source of light is provided with which is associated a reflector H. For directing the light from the source "I upon the unscanned side of the electrode or crystal target 32 a lens l4 and prism 2| are employed. Interposed in this light path is a polarizing screen I'G. Light which is reflected from the crystal or relay screen is pro.- jected upon a viewing screen 30 by means of a projection lens 20. This light path also includes a polarizing screen l8 anda lens element 18 is also employed which is common to both the proiected and the reflected light. The image is produced at the viewing screen 30 in accordance with the modulation of the-reflected light at the crystal plate 32, such modulation being controlled in accordance with the intensity of the scanning cathode ray beam. As explained above, when low television frame frequencies are employed, a certain amount of flickering may be observable at the viewing screen 30. In order to remove such flickering, or to render it imperceptible, as suggested above, a shutter disc 36 is employed, which is preferably positioned at the point of minimum beam cross-section. This shutter is driven by a synchronous motor 40, and since the television frame frequency is normally synchronized with the alternations of the power supply, the operation of the shutter and its rotation by the synchronous motor will naturally be synchronized with the television field frame frequency. The shutter 38, as shown in Figure 10, includes a plurality of apertures 38, which are separated one from another by means of the radial members 42 which obstruct the light beam and accordingly cause an interruption thereof at a frequency determined by the speed of the motor 40 and the number of radial shutter members 42.

In the form shown in Figure 2 a circular or annular light source I00. is employed, with which is associated an anular reflector 12a. This-conlight is directed upon the crystal plate 32 through the polarizing screen Ito, and the light reflected 1 Jected upon the viewing screen 30 by means of the projection lens 20. r

The form of the invention shown in Figure 2 is to be preferred, since it is entirely electrical in operation and does not include any moving elements or rotating masses. v

The light from a mercury discharge tube, which is not purely white light, can be converted through the use of the. known phosphorescent substances into a neutral projection light. However, in choosing the phosphorous substances for the walls of the gas discharge tube care should be taken to insure that the natural period of darkness of the gaseous discharge tube will not be bridged in part by after glow phenomena of the phosphorescent substances. This require ment can be readily complied with in view of the fact that phosphorous substances are known which, for all ranges of the spectrum, have a unscanned surface of the target, the intermittent rate of projection being greater than the scanning rate and bearing'a whole number ratio with respect thereto, -'and means includinga projection lens for directing the light reflected from the target area upon an observation screen.

2. A television receiving system including a cathode ray tube, a target area in said tube, means in said tube for electrically scanning the target area witha modulated beam of electrons, said target area comprising a crystal plate, means for intermittently and rapidly directing polarized light against the unscanned surface of the target, said means including a mercury vapor lamp operating from an alternating current supply,. the intermittent rate of projection being greater than the scanning rate and bearing a whole number ratio with respect thereto, and means including a projection lens for directing the light reflected from the target area upon a observation screen. r

3. A television receiving system includinga cathode ray tube, a target electrode in said tube, means in said tube for scanning one side of the target electrode with a beam of .electrons, an

annular source of light, means including a polarizing screen for directing light from the source upon the unscanned side of the target electrode, and means including a projection lens positioned axially with respect to the annular source of light for projecting light reflected from the target electrode upon a viewing screen, said target electrode including upon its scanned side a reflecting surface, said source of light comprising a mercury discharge lamp operating from an alternating current supply,the intermittent rate of projection being greater than the scanning rate and bearing a whole number ratio with respect thereto, whereby the, target electrode may be intermittently illuminated from said source.

' MANFRED vow ARDENNE. 

